A rotor system with multiple bearings is simply or multiply statically over-defined. This means that a change in the geometric position of a bearing relative to the shaft (or vice versa)—referred to in the following text as the alignment—results in a change in the bearing contact forces. Depending on the geometric configuration of the shaft and bearing system, even minor alignment changes result in major force changes on the bearings, and in consequence also in major changes to the bending stresses in the shaft system. Because of the load-dependent dynamic characteristics of journal bearings, the dynamic characteristics of a rotor system such as this likewise change with the contact force distribution.
Conversely, the knowledge of the static contact forces of the rotor system with multiple bearings allows assessment of the alignment of the bearings relative to the shaft system. The knowledge of the dynamic contact forces allows assessment of the oscillation state of the rotor system with multiple bearings.
Alignment errors can lead to increased bending stress loads on the shaft, and to excessive oscillations and bearing damage. High dynamic contact forces can likewise lead to bearing damage or consequential damage to adjacent or connected components, such as oil lines. The maximum permissible oscillation intensity is frequently specified by an operator. One feature of the static and dynamic contact forces is that they can vary during operation, both in the short term and in the long term. Short-term changes are caused by heating-up processes and load changes, for example. Long-term changes are caused by creepage deformation and seating phenomena, for example. Furthermore, changes in the static contact forces cannot be identified directly, since they become evident by different, often ambiguous changes in the operating behavior. The dynamic contact forces are generally determined by oscillation measurements, although the quantitative assessment thereof is feasible only if the system stiffnesses are known at the same time, and is therefore subject to considerable uncertainties.
Until now, the static bearing contact forces and the shaft alignment relative to the bearings have been determined by coupling the couplings to one another with no or with only a limited setting error. However, the changes during operation are not recorded directly in this way. In fact, indirect measurements, such as bearing temperature measurements, raised oil pressures and oscillations are measured, which allow an indirect conclusion to be drawn about the possible changes during operation in the shaft alignment relative to the bearings. During shut down periods, couplings are “broken”, and the alignment is determined by measurement of the coupling position error. Movement measurements carried out during operation on machine foundations and on stationary components likewise provide an indication of unacceptably major changes in the bearing contact forces which occur in the long term and short term.
The dynamic bearing contact faces are determined by oscillation measurements. Both relative and absolute oscillation measurements are carried out for this purpose. In the case of a relative oscillation measurement, the oscillation of the shaft is generally measured relative to the movement of a pick-up. The pick-up is attached to the bearing or to the bearing housing. In the case of an absolute oscillation measurement, the absolute movement is measured in three dimensions. In this case, the transmitter, that is to say a sensor or a measurement probe for determination of the absolute movement, is generally attached to the bearing or to the bearing housing. Estimates relating to contact estimates can be made by means of these measurements and further assumptions relating to the stiffness of the support.
However, the methods mentioned above have the common feature that, in some cases, they are subject to considerable uncertainties and provide only estimates of the bearing contact force to be determined.
FR 2 862 089 A1 describes a bearing having a sensor.